Stem rust (Puccinia graminis f. sp. tritici) of wheat is a major threat to global food security and necessitates the continued development of new stem rust resistant varieties. A stem rust race, Ug99 or TTKSK, first confirmed in Uganda in 1999 is virulent on many commercial varieties including those carrying the Sr31 resistance gene which hitherto had proven to be durable and widely cultivated (Jones et al., 1991; Bariana and McIntosh, 1993). Ug99 and its mutant derived races have spread to other African regions and the Middle East. Concerns over a potential epidemic should it reach the bread baskets of Asia has been a key driver of a global initiative to combat the food security threat posed by Ug99 and its lineage. Over 90% of wheat varieties in the proposed migration route of the pathogen are susceptible (Bariana and McIntosh, 1993). Global wheat breeding efforts to improve rust resistance are largely founded upon the repertoire of immune recognition specificities against the arsenal of wheat rust pathogen effectors that are embodied in the major resistance (R) genes found in the gene pool of wheat and its relatives. Combining different specific R genes capable of detecting a wide range of effectors is regarded as an effective strategy to contain rust epidemics in commercial agriculture.
Over 50 stem rust R genes that confer resistance at all growth stages have been catalogued in wheat inclusive of those introgressed from wild relatives. To date, none of these wheat stem rust R genes have been cloned. By contrast three wheat R genes (Lr1, Lr10 and Lr21) that provide protection against the wheat leaf rust fungus, Puccinia triticina, have been cloned (Somers et al., 2004; Hayden et al., 2008; Manly et al., 2001). The wheat stem rust R gene, Sr33, derived from the diploid D genome progenitor, Aegilops tauschii, (Kosambi, 1944) of common wheat (Triticum aestivum) exhibits a number of interesting features; it provides an intermediate resistance infection response against the Ug99 race and its lineage as well as all the commonly available rust isolates from diverse geographical regions (Kota et al., 2006). Efforts are underway to fully sequence the genomes and characterise effectors from wheat stem rust pathogens including Ug99 (Akhunov et al., 2010).
There is an urgent need for the identification of genes which confer at least some level of resistance to plants, especially wheat, against Puccinia graminis, such as the Ug99 group of races of Puccinia graminis f. sp. tritici. 